This invention relates to a ceramic heater which essentially consists of a sintered body of a nonoxide ceramic and a heating resistor which is embedded in the ceramic body as a heating element. The ceramic heater is used, for example, as a glow plug for a diesel engine or as an igniter for a gas or oil burner.
In conventional ceramic heaters the nonoxide ceramic body is usually formed of a silicon nitride (Si.sub.3 N.sub.4) based ceramic or an aluminum nitride (AlN) based ceramic. The material of the heating resistor is either a metal represented by tungsten and its alloys or a metal compound such as tungsten carbide (WC), titanium nitride or tantalum nitride. In the case of a metal resistor a coiled wire is often used. In the case of a metal compound resistor it is usual to form a so-called thick-film by a printing and firing process using a paste containing a powder of the resistor material.
As to known ceramic heaters, for example, JP 2-183718 A shows a glow plug having a heating resistor of tungsten in an aluminum nitride ceramic body which is coated with a silicon carbide film; JP 63-88777 A shows a ceramic heater having a WC based heating resistor in a silicon nitride or aluminum nitride based ceramic body; and JP 63-81787 A shows a ceramic heater having a TiN based heating resistor in an aluminum nitride based ceramic body.
Silicon nitride based ceramic is relatively low in heat conductivity (about 17 W/mK). Therefore, when the heating resistor in a heater body formed of this ceramic is energized a relatively long time runs before the surface temperature of the heating part reaches a sufficiently high level. Besides, in practical use of the heater it is necessary to keep the temperature of the silicon nitride based ceramic body below 1300.degree. C. If the temperature of the ceramic body exceeds 1300.degree. C. by the deliver of heat from the heating resistor or from a high temperature atmosphere in which the heater is used, the ceramic body is seriously and rapidly oxidized from the surface. Even in the air the progress of the oxidation of the ceramic body often results in oxidation and breakage of the heating resistor. In the case of a glow plug used in an engine the ceramic body becomes thin by erosion.
In a silicon nitride based ceramic there is a grain boundary phase which has a relatively low melting point (about 1400.degree. C.). When the heating resistor in the body of silicon nitride based ceramic is made of tungsten, tungsten alloy or tungsten carbide and kept energized for a long time until the temperature in the vicinity of the resistor nears 1500.degree. C., a reaction takes place between tungsten and the aforementioned grain boundary phase to form tungsten silicide WSi.sub.2 in the surface region of the resistor. As a result the heating resistor increases its resistance and becomes locally unconductive in an extreme case.
When the heating resistor in the body of silicon nitride based ceramic is made of a metal nitride such as TiN or TaN, there is a possibility that a fraction of the silicon nitride based ceramic undergoes electrolytic decomposition since an electrical potential difference occurs between the positive side and the negative side of the metal nitride resistor. If such decomposition occurs pores will appear in the ceramic body as a cause of lowering of the mechanical strength of the ceramic body, and the heating resistor might become locally unconductive.
Aluminum nitride based ceramic is relatively high in heat conductivity (about 170 W/mK). As the material of the body of a ceramic heater this property is favorable for a rapid rise in the temperature of the surface of the heating part of the heater body. However, in this case there arises a different problem. Usually a part of the ceramic body is used as a supporting part by not embedding the heating resistor in that part, and electrical terminals of the ceramic heater and external leads are connected in the supporting part. When the ceramic heater having an aluminum nitride based ceramic body is operated so as to keep the temperature of the heating part about 1300.degree. C. or above the temperature of the supporting part of the ceramic body rises up to about 800.degree. C. in a short time. Therefore there is a possibility of oxidation of the soldered connections of the electrical terminals and resultant degradation of the electrical connection. Besides, the high heat conductivity of the ceramic body causes easy liberation of heat from the supporting part of the ceramic body, whereby the power consumption of the heater increases.